When welding transparent resin films to each other, the resin films are sandwiched between blocks. Thus, the resin films are stacked on top of each other with a predetermined pressing force. An absorber for absorbing a laser beam is applied to an interface between the resin films beforehand. A laser beam is emitted from outside the outline of the blocks toward the interface in a direction perpendicular to the front surfaces of the resin films. The absorber absorbs the laser beam. As a result, the resin films are heated to the melting point of the resin films at the interface, and the resin films melt. Thereafter, the resin films are cooled, so that the resin films become welded to each other. (See Japanese Laid-open Patent Publication Nos. 2000-334589, 9-510930, 2007-175901, 2009-12239, and 2003-530240.)
The laser beam melts the resin films while the laser beam is moved parallel to the front surfaces of the resin films. Because the laser beam enters the resin films from a direction perpendicular to the front surfaces of the resin films, the blocks are disposed so as to apply a pressing force to the resin films in an area that does not include positions at which the laser beam enters the resin films. As a result, a sufficient pressing force is not applied to the resin films at melting positions of the resin films. The resin films do not sufficiently contact each other at the melting position. Moreover, it is necessary to apply the absorber to the interface between the resin films beforehand. The cost of welding increases due to the use of the absorber.